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BIG PHYSICS, BIG QUESTIONS –

The ultimate in eating disorders

By Philip Cohen

THE sexual etiquette of female spiders, who often eat their suitors, has long appalled the faint-hearted. Scientists had always assumed that the females derive some advantage from this arrangement, but now two biologists claim that this bizarre behaviour is a big mistake for both sexes.

Male spiders are the obvious losers from this sexual cannibalism as they are often eaten before they have had a chance to mate. Females have been thought to be acting in their own interests&colon; if they are hungry, the theory goes, then a male is seen as lunch, rather than a lover. But if the female is already well fed, then some after dinner sex is on the cards.

To test this theory, Göran Arnqvist and Stefan Henriksson of Umeå University studied adult fishing spiders. These spiders grow to a length of more than 3 centimetres and can eat tadpoles and small fish. The researchers fed some females hearty meals, and put others on a diet. Against all predictions, both groups of females were equally likely to eat their suitors. Virgin females devoured their beaus as often as those that were already fertilised.

When the researchers looked closer, they found that the females’ appetites were definitely undermining their reproductive interest&colon; a third of females had less than half of their eggs fertilised, even though they had encountered enough males to have guaranteed complete fertilisation.

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So if cannibalism benefits neither males nor females, why did it evolve? The Swedish researchers argue that the behaviour is a hangover from the females’ voracious youth. Spiders usually produce only one batch of eggs per lifetime. The number of eggs in that batch is determined by the size of the female, which in turn depends upon the amount of food she ate as a juvenile.

Arnqvist and Henriksson note that juvenile female spiders will even eat one another, which means that only the most voracious will reach adulthood. “There’s incredibly strong selection for females to be aggressive feeders,” says Arnqvist. The researchers believe that this aggression is governed by a single hormone, and so cannot easily be shrugged off when the time comes to mate. “It’s a very interesting result,” says Daniel Rubenstein of Princeton University in New Jersey. “It shows that what’s a good idea when you are young isn’t necessarily good when you are older.”

Arnqvist and Henriksson will describe their results in full in a future issue of the journal Evolutionary Ecology.